docs: def MergeWaveSets(waveset1, waveset2):
      """Return the union of the two wavelength sets.
  
      The union is computed using `numpy.union1d`, unless one or
      both of them is `None`.
  
      The merged result may sometimes contain numbers which are nearly
      equal but differ at levels as small as 1E-14. Having values this
      close together can cause problems due to effectively duplicate
      wavelength values. Therefore, wavelength values having differences
      smaller than or equal to ``pysynphot.spectrum.MERGETHRESH``
      (defaults to 1E-12) are considered as the same.
  
      Parameters
      ----------
      waveset1, waveset2 : array_like or `None`
          Wavelength sets to combine.
  
      Returns
      -------
      MergedWaveSet : array_like or `None`
          Merged wavelength set. It is `None` if both inputs are such.
  
      """
      if waveset1 is None and waveset2 is not None:
          MergedWaveSet = waveset2
      elif waveset2 is None and waveset1 is not None:
          MergedWaveSet = waveset1
      elif waveset1 is None and waveset2 is None:
          MergedWaveSet = None
      else:
          MergedWaveSet = N.union1d(waveset1, waveset2)
  
          # The merged wave sets may sometimes contain numbers which are nearly
          # equal but differ at levels as small as 1e-14. Having values this
          # close together can cause problems down the line so here we test
          # whether any such small differences are present, with a small
          # difference defined as less than MERGETHRESH.
          #
          # If small differences are present we make a copy of the union'ed array
          # with the lower of the close together pairs removed.
          delta = MergedWaveSet[1:] - MergedWaveSet[:-1]
  
          if not (delta > MERGETHRESH).all():
              newlen = len(delta[delta > MERGETHRESH]) + 1
              newmerged = N.zeros(newlen, dtype=MergedWaveSet.dtype)
              newmerged[:-1] = MergedWaveSet[:-1][delta > MERGETHRESH]
              newmerged[-1] = MergedWaveSet[-1]
  
              MergedWaveSet = newmerged
  
      return MergedWaveSet